The invention is related to continuously variable transmission unit, having two pulley sets which each comprise a pair of discs, a support shaft and a screw mechanism, each screw mechanism being concentric with respect to the respective support shaft for moving the discs of the respective pulley set towards and from each other, each pair of discs enclosing a V-shaped groove, an endless flexible element stretching between the pulley sets and accommodated in the grooves thereof, the screw mechanism being controllable by a control means.
Such continuously variable transmission unit is disclosed in EP-A-582307. The prior art continuously variable transmission unit has the disadvantage that its overall dimensions are large, which is undesirable in particular in the automotive field.
The object of the invention is to provide a continuously variable transmission unit which does not have this disadvantage. The object is achieved in that each screw mechanism is accommodated in a bore in the corresponding support shaft. The screw mechanism engages at least one of the discs through at least one longitudinal aperture extending from the bore towards the outer circumference of the support shaft.
By positioning the screw mechanism concentric inside the shaft, the overall dimensions of the unit are reduced. Moreover, the adjusting clamping force exerted by the screw mechanism is also central with respect to the discs, which provides a direct drive without play. Thereby, the displacements of the discs can be controlled in a well-defined way, which enhances the accuracy of the transmission ratio.
According to a preferred embodimnent, at least one of the pulleys has a screw mechanism comprising two screw actuators which have mutually opposite screw thread angles, each screw actuator engaging a respective disc. The discs of the pulleys can then be displaced symmetrically. As a result, no transverse displacements are imposed on the belt. Thus, the belt is loaded symmetrically with respect to the plane of symmetry of the pulley sets, leading to a longer service life of the transmission.
Moreover, as a result of the circumstance that both discs can be moved, the total displacement of each disc is only half the distance over which the movable disc is to be displaced in case the other disc is fixed, as according to EP-A-582307. Thus, relatively rapid changes in transmission ratio are possible, without the displacement speed of the discs becoming too high.
Additionally the advantage is obtained that the components of each pulley set can be identical which reduces the cost.
Reference is made to the continuously variable transmission unit as disclosed in SU-A-732085. The unit has pulleys, the discs of which each engage a central shaft by means of a screw mechanism having threads with opposite pitch. By means of springs, the discs of each pulley are constantly urged towards each other. A relative displacement of the sheaves is obtained as a result of increased loadings on the belt.
The unit lacks a drive mechanism, nor does the screw mechanism constitute an actuator for actively influencing the relative position of the discs.
Preferably, each disc of the at least one pulley is slidably but non-rotatably accommodated on the support shaft, the screw actuators engaging opposite ends of the support shaft. Each screw actuator engages the support shaft by means of a four-point angular contact ball bearing with different load angles.
Each screw actuator engages the support shaft by means of a support bearing. The bearings may be carried out as four-point angular contact ball bearings, preferably of asymmetric design. For reasons of a higher load bearing capacity and compactness, a full complement bearing may be used.
The outer ring of at least one support bearing is integrated with the shaft.
Each screw actuator comprises a nut and a screw, the nuts each engaging a disc, the screws engaging the support shaft.
The screw actuator may be either of a ball screw or of a roller screw type. In the latter case, the radial dimensions of the screw actuator are relatively small, resulting in reduced overall dimensions of the complete unit.
The nuts of the screw actuators can be accommodated on a common drive shaft extending through the support shaft.
The support shaft is supported by means of support bearings with respect to a housing, the nut of each screw actuator having at least one radial extension engaging the corresponding disc. In that case, the support shaft has at least one slot through which an extension extends.
The support shaft at one end is connected to an input shaft or an output shaft, and at the other end to a drive mechanism for controlling the transmission ratio.